Positive electrode active material and method of producing positive electrode active material

ABSTRACT

A method of producing a positive electrode active material, the method includes: contacting first particles that contain a lithium transition metal composite oxide with a solution containing sodium ions to obtain second particles containing the lithium transition metal composite oxide and sodium element, wherein the lithium transition metal composite oxide has a layered structure and a composition ratio of a number of moles of nickel to a total number of moles of metals other than lithium in a range of from 0.7 to less than 1; mixing the second particles and a boron compound to obtain a mixture; and heat-treating the mixture at a temperature in a range of from 100° C. to 450° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2019-129204, filed on Jul. 11, 2019, and Japanese Patent Application No.2020-097755, filed on Jun. 4, 2020, the contents of which are herebyincorporated by reference in their entireties.

BACKGROUND

The present disclosure relates to a positive electrode active materialand a method of producing the positive electrode active material.

A positive electrode active material for a non-aqueous electrolytesecondary battery contains a lithium-transition metal composite oxide,such as lithium cobaltate, lithium nickelate, and nickel cobalt lithiummanganate. A non-aqueous electrolyte secondary battery containing alithium nickel-based composite oxide with a higher ratio of nickel, inplace of cobalt, a scarce resource, advantageously has a higherelectrical charge-discharge capacity per unit weight. However, it is noteasy to synthesize a lithium nickel-based composite oxide, and a part ofthe raw materials unreacted in the synthesis may remain as residualalkaline component. Such residual alkaline component may cause, forexample, slurry thickening during electrode fabrication, or gasgeneration during charging. However, reducing such residual alkalinecomponent by, for example, water-washing may worsen the cyclecharacteristics of the battery to be produced.

In view of the above, a technique for improving the cyclecharacteristics of a non-aqueous electrolyte secondary battery to beproduced has been proposed by water-washing a lithium nickel-basedcomposite oxide, and then contacting the composite oxide with an aqueoussolution of sulfate to cause sulfate to be present on the surface of theprimary particles (refer to, for example, Japanese Patent PublicationNo. 2011-124086). Another proposed technique for improving the cyclecharacteristics includes water-washing a lithium nickelate-based lithiumtransition metal composite oxide, mixing the composite oxide with aboron compound, and heat-treating the mixture (refer to, for example,Japanese Patent Publication No. 2015-088343).

SUMMARY

A first aspect is a method of producing a positive electrode activematerial with good cycle characteristics.

A method of producing a positive electrode active material, the methodincludes: contacting first particles that contain a lithium transitionmetal composite oxide with a solution containing sodium ions to obtainsecond particles containing the lithium transition metal composite oxideand sodium element, wherein the lithium transition metal composite oxidehas a layered structure and a composition ratio of a number of moles ofnickel to a total number of moles of metals other than lithium in arange of from 0.7 to less than 1; mixing the second particles and aboron compound to obtain a mixture; and heat-treating the mixture at atemperature in a range of from 100° C. to 450° C.

A second aspect is a positive electrode active material containingsecondary particles each composed of an aggregate of primary particlesthat contain a lithium transition metal composite oxide having a layeredstructure and a composition ratio of the number of moles of nickel tothe total number of moles of metals other than lithium of from 0.7 toless than 1. In the positive electrode active material, a compoundcontaining boron is attached on at least a part of the surfaces of theprimary particles, a compound containing sodium is present in at least apart of the particle boundaries of the secondary particles, and a valueobtained by dividing a standard deviation of detection amounts of boronelement in any three regions in a cross-sectional surface of thesecondary particles by an average of the detection amounts is less than0.18.

A third aspect is a non-aqueous electrolyte secondary battery containingthe positive electrode active material according to the presentdisclosure in its positive electrode.

A fourth aspect is an electrode for a non-aqueous electrolyte secondarybattery containing a current collector and a positive electrode activematerial layer arranged on the current collector. The positive electrodeactive material layer contains the positive electrode active materialaccording to the present disclosure and has a density in a range of 2.8g/cm³ to 3.7 g/cm³.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example image showing BO²⁻ dispersion in a positiveelectrode active material particle of Example 5.

FIG. 2 is an example image showing BO²⁻ dispersion of a positiveelectrode active material particle of Comparative Example 5.

FIG. 3 is an example image showing BO²⁻ dispersion of a positiveelectrode active material particle of Comparative Example 6.

FIG. 4 is an equivalent circuit model used in AC impedance measurement.

DETAILED DESCRIPTION

According to an aspect of the present disclosure, a method of producinga positive electrode active material with good cycle characteristics maybe provided.

As used herein, the term “step” means not only an independent step butalso a step which cannot be clearly distinguished from the other stepsbut can achieve the desired object. For the amount of each componentcontained in a composition, when a plurality of substances correspondingto the component are present in the composition, the amount of thecomponent means the total amount of the corresponding substances presentin the composition unless otherwise specified. The present inventionwill now be described in detail by means of embodiments. However, theembodiments shown below are mere examples of the positive electrodeactive material and the method of producing the positive electrodeactive material for embodying the technical concept of the presentinvention, and the present invention is not limited to the positiveelectrode active material and the method of producing the positiveelectrode active material described below.

Method of Producing Positive Electrode Active Material

The method of producing a positive electrode active material may includea washing step of contacting first particles that contain a lithiumtransition metal composite oxide having a layered structure and acomposition ratio of the number of moles of nickel to the total numberof moles of metals other than lithium in a range of from 0.7 to lessthan 1 with a solution containing sodium ions to obtain second particlescontaining the lithium transition metal composite oxide and sodiumelement, a mixing step of mixing the second particles and a boroncompound to obtain a mixture, and a heat-treatment step of heat-treatingthe mixture at a temperature in a range of from 100° C. to 450° C. Themethod may further include other steps including a providing step ofproviding the first particles as appropriate.

The positive electrode active material obtained by washing a lithiumtransition metal composite oxide having a ratio of the number of molesof nickel in a range of 0.7 and less than 1 with a sodium ion-containingaqueous solution, and then heat-treating the lithium transition metalcomposite oxide with a boron compound can achieve good cyclecharacteristics when used in a non-aqueous electrolyte secondarybattery. This is seemingly partly because the presence of sodium in theparticle boundaries of the secondary particles, which contain thelithium transition metal composite oxide for forming the positiveelectrode active material, allows boron to be uniformly dispersed acrossthe particle boundaries of the lithium transition metal composite oxideparticles.

Providing Step

In the providing step, the first particles containing layer-structuredlithium transition metal composite oxide are provided. The lithiumtransition metal composite oxide to be contained in the first particlescontains nickel in its composition, and has a composition ratio of thenumber of moles of nickel to the total number of moles of metals otherthan lithium in a range of 0.7 to less than 1. The first particles maybe appropriately selected from commercially available products, or maybe provided by the provision method as described below.

The provision method of the first particles may include, for example, aprecursor provision step of providing precursors, and a synthesis stepof synthesizing the first particles containing a lithium transitionmetal composite oxide from the precursors and a lithium compound. Thefirst particles may be formed, for example, as secondary particlescomposed of a plurality of primary particles containing the lithiumtransition metal composite oxide.

In the precursor provision step, a precursor containing anickel-containing composite oxide is provided. The precursor may beappropriately selected from commercially available products, or may beprovided by providing a nickel-containing composite oxide having adesired composition using a common method. Examples of the precursorinclude nickel-containing composite oxides as well as composite oxidescontaining nickel and metals other than nickel (for example, Co, Mn, Al,Ti, and Nb).

A nickel-containing composite oxide having a desired composition may beobtained by, for example, a method of mixing raw material compounds (forexample, hydroxide and carbonic acid compound) in a manner to meet atarget composition, and heat-treating the mixture to decompose themixture into a nickel-containing composite oxide, or by thecoprecipitation method of providing a solution in which raw materialcompounds are dissolved, forming a precursor precipitate having a targetcomposition through, for example, temperature adjustment, pH adjustment,and introduction of a complexing agent, and heat-treating the precursorprecipitate. An example method of producing a nickel-containingcomposite oxide (hereinafter also simply referred to as “compositeoxide”) will now be described.

The coprecipitation method of obtaining a composite oxide may include aseed generation step of adjusting pH and others of a mixture solutioncontaining metal ions in a desired ratio to generate seed crystals, acrystallization step of growing the generated seed crystals to obtain acomposite hydroxide having desired properties, and a compositehydroxide-obtaining step of heat-treating the resulting compositehydroxide. For the details of such a method of obtaining a compositeoxide, refer to, for example, Japanese Patent Publications No.2003-292322 and Japanese Patent Publication No. 2011-116580(Specification of U.S. Patent Publication No. 2012/270107).

In the seed generation step, a mixture solution containing nickel ionsat a desired ratio may be adjusted to have a pH of, for example, in arange of from 11 to 13 to provide a liquid medium containing seedcrystals. The seed crystals may, for example, contain hydroxidecontaining nickel in a desired ratio. The mixture solution may beprovided by dissolving a nickel salt in water in a desired ratio.Examples of the nickel salt include sulfate, nitrate, and hydrochloride.In addition to the nickel salt, the mixture solution may contain othermetal salts in a desired ratio as appropriate. The seed generation stepmay be carried out at a temperature of, for example, from 40° C. to 80°C., and in a low-oxidation atmosphere with an oxygen concentration of,for example, 10% by volume or less.

In the crystallization step, the generated seed crystals are grown toobtain a nickel-containing precursor precipitate with desiredproperties. The seed crystals may be grown by, for example, adding amixture solution containing nickel ions and other metal ions asappropriate to the liquid medium containing the seed crystals whilemaintaining the pH of the liquid medium in a range of, for example, from7 to 12.5, and preferably 7.5 to 12. The mixture solution is added in aperiod of, for example, from 1 hour to 24 hours, and preferably from 3hours to 18 hours. The crystallization step may be carried out at atemperature of, for example, 40° C. to 80° C. in the same atmosphere asthe seed generation step.

In the seed generation and crystallization steps, the pH may be adjustedusing an aqueous acidic solution, such as an aqueous solution ofsulfuric acid and an aqueous solution of nitric acid, or an aqueousalkaline solution, such as an aqueous solution of sodium hydroxide andan ammonia water.

In the crystallization step, it is desirable to control the particlediameter of the precursor precipitate. The particle diameter of theprecursor precipitate may be controlled by adjusting, for example, thetemperature and pH of the reaction site, and the stirring speed. Theseconditions may be appropriately adjusted in accordance with the actualconditions including the shape of the container containing the reactionsite, the starting materials, and the speed at which the startingmaterials are supplied into the reaction site. The particle diameter ofthe precursor precipitate may also be controlled by adjusting, forexample, the aging time since the precursor precipitate starts toprecipitate and the stirring speed in accordance with the actualconditions because the particle growth rate, the particle shape, and soforth vary depending on the shape of the reaction container.

In the composite oxide-obtaining step, the precursor precipitatecontaining the composite hydroxide obtained in the crystallization stepis heat-treated to obtain a composite oxide. The heat treatment may becarried out by heating the composite hydroxide at a temperature of, forexample, 500° C. or less, and preferably 350° C. or less. The heattreatment temperature is, for example, 100° C. or more, and preferably200° C. or more. The heat treatment period is, for example, 0.5 hours to48 hours, and preferably 5 hours to 24 hours. The heat treatmentatmosphere may be the air or an oxygen-containing atmosphere. The heattreatment may be carried out using, for example, a box furnace, a rotarykiln furnace, a pusher furnace, and a roller hearth kiln furnace.

The resulting composite oxide may contain other metal elements inaddition to nickel. Examples of the other metals include Co, Mn, Al, Ti,and Nb, and at least one selected from the group consisting of thesemetals is preferable, and at least one selected from the groupconsisting of Co, Mn, and Al is more preferable. A composite oxidecontaining other metals may be obtained by preparing a mixture solutionfor forming a precursor precipitate to contain other metal ions in adesired ratio. This can allow the precursor precipitate to contain othermetals in addition to nickel, and heat-treating the precursorprecipitate can yield a composite oxide having a desired composition.

The composite oxide may have an average particle diameter of, forexample, from 2 μm to 30 μm, and preferably from 3 μm to 25 μm. Theaverage particle diameter of the composite oxide is a volume meandiameter defined as a value corresponding to 50% volume accumulationfrom the small particle diameter side in a volume distribution measuredusing a laser scattering method.

In the synthesis step of synthesizing the first particles containing alithium transition metal composite oxide, a mixture containing lithiumobtained by mixing a composite oxide described above and a lithiumcompound may be heat-treated at a temperature of from 550° C. to 1000°C. to obtain a heat-treated product. The heat-treated product thusobtained may have a layered structure, and contain a nickel-containinglithium transition metal composite oxide.

Examples of the lithium compound to be mixed with the composite oxideinclude lithium hydroxide, lithium carbonate, and lithium oxide. Thelithium compound to be mixed may have a particle diameter in terms ofvolume mean particle diameter in a range of, for example, from 0.1 μm to100 μm, and preferably from 2 μm to 20 μm.

The ratio of the total number of moles of lithium to the total number ofmoles of metal elements contained in the composite oxide in the mixtureis in a range of, for example, from 0.95 to 1.2. The composite oxide andthe lithium compound may be mixed using, for example, a high-speed shearmixer.

The mixture may further contain other metals in addition to lithium andmetal elements contained in the composite oxide. Examples of the othermetal elements include Al, Si, Zr, Ti, Mg, Ta, Nb, Mo, and W, and atleast one selected from the group consisting of these metal elements ispreferable. When the mixture contains, for example, W, Nb, or the likeas the other metal elements, it may be desirable for improvement ofoutput characteristics. When the mixture contains, for example, Al, Zr,or the like, it may be desirable for further improvement of cyclecharacteristics. When the mixture contains, for example, Ti, Si, or thelike, it may be desirable for further improvement of cyclecharacteristics under high voltage. When containing other metalelements, the mixture may be obtained by mixing metal elementalsubstances or metal compounds of other metals together with thecomposite oxide and the lithium compound. Examples of the metalcompounds containing other metal elements include oxides, hydroxides,chlorides, nitrides, carbonates, sulfates, nitrates, acetates, andoxalates.

When containing other metal elements, the mixture may have a ratio ofthe total number of moles of metal elements contained in the compositeoxide to the total number of moles of other metals in the mixture in arange of, for example, from 1:0.0001 to 1:0.1, preferably from 1:0.0005to 1:0.03, or from 1:0.001 to 1:0.01.

The mixture may be heat-treated at a temperature in a range of, forexample, from 550° C. to 1000° C., preferably from 600° C. to 950° C.,and more preferably from 700° C. to 950° C. Although the mixture may beheated at a single temperature, the mixture is preferably heated atmultiple temperatures in a stepwise manner to improve electricaldischarge capacity at a high voltage. When heat-treated at multipletemperatures, for example, the mixture is heat-treated at a firsttemperature for a predetermined time, then the temperature is increasedto a second temperature, and the mixture is heat-treated at the secondtemperature for a predetermined time. The first temperature may be, forexample, from 200° C. to 600° C., and preferably from 400° C. to 500° C.The second temperature may be, for example, from 600° C. to 900° C., andpreferably from 650° C. to 750° C. The heat treatment time may be, forexample, 0.5 hour to 48 hours. When the mixture is heat-treated atmultiple temperatures, the period for each heating may be from 0.2 hourto 47 hours.

The mixture may be heat-treated in the air or in an oxygen-containingatmosphere. The mixture may be heat-treated in, for example, a boxfurnace, a rotary kiln furnace, a pusher furnace, or a roller hearthkiln furnace.

The lithium transition metal composite oxide thus obtained has acomposition ratio of the number of moles of nickel to the total numberof moles of metals other than lithium in a range of from 0.7 to lessthan 1, preferably from 0.7 to 0.95, more preferably from 0.75 to 0.95,and still more preferably from 0.8 to 0.95. The lithium transition metalcomposite oxide may contain cobalt. When containing cobalt, the lithiumtransition metal composite oxide has a ratio of the number of moles ofcobalt to the total number of moles of metals other than lithium in arange of, for example, more than 0 to 0.3, and preferably from 0.02 to0.2. The lithium transition metal composite oxide may contain manganese.When containing manganese, the lithium transition metal composite oxidehas a ratio of the number of moles of manganese to the total number ofmoles of metals other than lithium in a range of, for example, more than0 to 0.3, preferably more than 0 to 0.15, and more preferably from 0.01to 0.15. The lithium transition metal composite oxide may containaluminium. When containing aluminium, the lithium transition metalcomposite oxide has a ratio of the number of moles of aluminium to thetotal number of moles of metals other than lithium in a range of, forexample, more than 0 to 0.1, preferably more than 0 to 0.05, and morepreferably from 0.01 to 0.04. When containing at least one of manganeseand aluminium, the lithium transition metal composite oxide has a ratioof the number of moles of manganese and aluminium to the total number ofmoles of metals other than lithium in a range of, for example, more than0 to 0.3, preferably more than 0 to 0.25, and more preferably from 0.01to 0.15.

The lithium transition metal composite oxide can have a compositionrepresented by, for example, formula (1):Li_((1+p))Ni_((1−x−y−z−w))Co_(x)Mn_(y)Al_(z)M_(w)O₂  (1)

In the formula, −0.05≤p≤0.2, 0<x+y+z+w≤0.3, 0≤x≤0.3, 0≤y≤0.3, 0≤z≤0.1,and 0≤w≤0.03; and M is at least one selected from the group consistingof Zr, Ti, Mg, Ta, Nb, Mo, and W.

The first particles containing the lithium transition metal compositeoxide may have a volume mean particle diameter in a range of, forexample, from 2 μm to 30 μm, and preferably from 3 μm to 25 μm.

Washing Step

In the washing step, the first particles containing lithium transitionmetal composite oxide are contacted with a solution containing sodiumions (hereinafter also referred to as a washing liquid) to obtain secondparticles containing the lithium transition metal composite oxide andsodium element. The product obtained through contact with the washingliquid may undergo, for example, dewatering or drying treatment asappropriate. The washing step may be to remove, for example, at least apart of alkaline component of the raw materials left unreacted in thefirst particles.

The solution containing sodium ions only needs to contain at leastsodium ions and water. The solution containing sodium ions may beprovided by, for example, dissolving a sodium salt in a solvent.Examples of the sodium salt include sodium sulfate and sodium hydroxide,and at least one selected from the group consisting of these ispreferable, and a solution containing at least sodium sulfate is morepreferable. The solvent may contain, for example, water alone, and mayfurther contain a water soluble organic solvent, such as alcohol, asappropriate in addition to water. The sodium ion content of the washingliquid may be, for example, from 0.01 mol/L to 2.0 mol/L, preferablyfrom 0.05 mol/L to 2.0 mol/L, more preferably from 0.1 mol/L to 1.5mol/L, still more preferably from 0.15 mol/L to 1.0 mol/L, andespecially preferably from 0.15 mol/L to 0.6 mol/L.

The washing liquid may contain other metal ions in addition to sodium.Examples of the metal ions other than sodium include alkali metal ions,such as lithium ions and potassium ions, and alkaline earth metal ions,such as magnesium ions. When the washing liquid contains other metalions in addition to sodium ions, the other metal ion content of thewashing liquid may be, for example, 0.1 mol/L or less, and preferablyless than 0.01 mol/L.

The temperature at which the first particles and the washing liquid arecontacted with each other may be, for example, in a range of from 5° C.to 60° C., and preferably from 10° C. to 40° C. The contact period maybe, for example, in a range of from 1 min to 2 hours, and preferablyfrom 5 min to 30 min. The amount of the washing liquid to be used forthe contact may be, for example, in a range of from 0.25 times to 10times, preferably 0.5 time to 4 times of the mass of the firstparticles.

The first particles and the washing liquid may be contacted by supplyingthe first particles into the washing liquid to provide a slurry. When aslurry is formed by the contact, the solid concentration of the firstparticles in the slurry may be, for example, in a range of from 10% bymass to 80% by mass, and preferably from 20% by mass to 60% by mass.Also, the contact may be carried out by passing the washing liquidthrough the first particles being held on a filter, or may be carriedout by washing the first particles with, for example, pure water,dewatering the first particles to obtain a dewatered cake, and passingthe washing liquid through the dewatered cake. When the contact iscarried out by washing the first particles with, for example, purewater, dewatering the first particles to obtain a dewatered cake, andpassing the washing liquid through the dewatered cake, the total amountof the pure water and washing liquid to be used for the contact may be,for example, in a range of from 0.25 times to 10 times, preferably 0.5time to 4 times of the mass of the first particles. However, a solutioncontaining sodium ions (for example, a sodium sulfate solution)dissolves residual alkaline (for example, a lithium carbonate) betterthan water, and thus washes residual alkaline more easily. Thus,contacting the first particles with a washing liquid is preferablycarried out rather than washing with pure water in terms of reducingdamage on the lithium transition metal composite oxide. It is morepreferable not to perform washing with pure water.

The second particles obtained in the washing step contain asodium-containing compound in addition to the lithium transition metalcomposite oxide. The sodium-containing compound may be present in, forexample, the particle boundaries of the secondary particles, which arecomposed of the primary particles containing the lithium transitionmetal composite oxide. The second particles contain thesodium-containing compound in an amount in a range of, for example, from100 ppm to 1400 ppm, preferably from 150 ppm to 1300 ppm, morepreferably from 150 ppm to 1200 ppm, still more preferably from 200 ppmto 1000 ppm, and particularly preferably from 300 ppm to 1000 ppm interms of sodium element. When the amount of the sodium-containingcompound is within the above-described range, the resistance componentduring electrical charge and discharge is sufficiently reduced. Theratio of the sodium-containing compound in the second particles may beadjusted by, for example, adjusting the sodium ion concentration of thewashing liquid, and the amount of water adhered to the dewatered cake.

The second particles obtained in the washing step can undergo dryingtreatment. In the drying treatment, it suffices that at least partiallyremove water adhered to the second particles. This may be carried outby, for example, heat drying, air drying, and vacuum drying. The dryingtemperature during heat drying is sufficient if water contained in thesecond particles is adequately removed. The drying temperature is, forexample, from 80° C. to 300° C., and preferably from 100° C. to 250° C.When the drying temperature is within this range, elution of lithiuminto the adhered water can sufficiently be reduced. Also, this canreduce crystal structure disintegration on the particle surfaces, andinhibit lowering of the electrical charge-discharge capacitysufficiently. The drying time may be appropriately selected inaccordance with the amount of water contained in the second particles.The drying time is, for example, from 1 hour to 10 hours. The amount ofwater contained in the second particles after drying treatment is, forexample, 0.2% by mass or less, and preferably 0.1% by mass or less.

The degree of washing in the washing step may be checked by the lithiumcontent, residual alkaline component, and specific surface area of thesecond particles. In general, when the second particles have a smallspecific surface area, for example, particle cracking and elution ofelements from the lithium and the composite oxide can be sufficientlyreduced. This may improve cycle characteristics. Also, when the secondparticles have a certain size of specific surface area, the residualalkaline component can be sufficiently reduced. An aqueous solution ofsodium salt, such as sodium sulfate, as a washing liquid can dissolvelithium salt better than, for example, pure water or an aqueous solutionof lithium salt, and thus needs a less amount of liquid for removingalkaline. As a result, the second particles have a smaller specificsurface area, which can reduce excessive elution of lithium from thesecond particles.

The second particles obtained from the washing step have a specificsurface area in a range of, for example, from 0.5 m²/g to 4 m²/g,preferably from 1.0 m²/g to 3.0 m²/g, more preferably from 1.0 m²/g to1.6 m²/g, and still more preferably from 1.0 m²/g to 1.4 m²/g. Thespecific surface area may be measured by the BET method.

Mixing Step

In the mixing step, the second particles and a boron compound are mixedto obtain a mixture. The mixing of the second particles and a boroncompound may be carried out either by a dry or wet method. The mixing ofthe second particles may be carried out by using, for example, a supermixer. In the mixing step, the second particles may be mixed withelemental substances, alloy or metal compound of other metal element inaddition to a boron compound. The other element includes Al, Si, Zr, Ti,Mg, Ta, Nb, Mo, and W, and at least one selected from the groupconsisting of these metal elements is preferable.

The boron compound may be at least one selected from the groupconsisting of boron oxides, boron oxo acid, and boron oxo acid salt.More specific examples of the boron compound include lithium tetraborate(Li₂B₄O₇), ammonium pentaborate (NH₄B₅O₈), orthoboric acid (H₃BO₃, orordinary boric acid), lithium metaborate (LiBO₂), and boron oxide(B₂O₃), and at least one selected from the group consisting of these ispreferable. Among these examples, orthoboric acid is more preferablefrom the aspect of cost.

The boron compound may be mixed in a solid state or in the form of asolution with the second particles. When used in a solid state, theboron compound has a volume mean particle diameter of, for example, from1 μm to 60 μm, and preferably from 10 μm to 30 μm.

The mixture has a boron compound content, as a percentage of the numberof moles of boron element to the total number of moles of metals otherthan lithium in the lithium transition metal composite oxide, of, forexample, from 0.1% by mole to 2% by mole, preferably from 0.1% by moleto 1.5% by mole, and more preferably from 0.1% by mole to 1.2% by mole.

Heat Treatment Step

In the heat-treatment step, the mixture is heat-treated, for example, ata temperature in a range of from 100° C. to 450° C. to obtain a positiveelectrode active material. The heat-treating temperature may be in arange of from 200° C. to 400° C., preferably from 220° C. to 350° C.,and more preferably from 250° C. to 350° C. The heat-treatingtemperature set higher than the drying treatment temperature may furtherimprove electrical charge-discharge capacity. The heat-treatingatmosphere can be an oxygen-containing atmosphere or the air. Theheat-treating time is, for example, from 1 hour to 20 hours, andpreferably from 5 hours to 10 hours. The heat-treated product obtainedthrough the heat-treatment step may undergo, for example, crushingtreatment and classifying treatment.

The second particles after the washing step may have a lithium-deficientregion near the surfaces. In the lithium-deficient region, desorptionand insertion of lithium ions may be inhibited. However, mixing a boroncompound with the second particles after the washing step, andheat-treating the mixture can be believed to compensate such lithiumdeficit, reduce inhibition of desorption and insertion of lithium ions,and improve electrical charge and discharge characteristics and cyclecharacteristics.

Positive Electrode Active Material

The positive electrode active material may contain secondary particlesbeing composed of a plurality of primary particles containing a lithiumtransition metal composite oxide having a layered structure and acomposition ratio of the number of moles of nickel to the total numberof moles of metals other than lithium in a range of from 0.7 to lessthan 1. A compound containing boron may be attached on at least a partof the surfaces of the primary particles. A compound containing sodiummay be present in at least a part of the particle boundaries of thesecondary particles. A value (σ1/t1), or a coefficient of variation(CV), obtained by dividing a standard deviation σ1 of detection amountsof boron element in any three regions in a cross-sectional surface ofthe secondary particles by an average t1 of the detection amounts isless than 0.18.

The positive electrode active material contains the secondary particlesbeing composed of a plurality of the primary particles. The primaryparticles contain lithium transition metal composite oxide and has astructure on the surfaces of which a compound containing boron isattached. This composition enables a non-aqueous electrolyte secondarybattery containing the positive electrode active material to haveimproved electrical charge and discharge characteristics and cyclecharacteristics. Also, a compound containing sodium in the particleboundaries of the secondary particle allows the compound containingboron to be uniformly dispersed across the secondary particles. Thisstructure is believed to achieve good electrical charge and dischargecharacteristics and cycle characteristics. The positive electrode activematerial can be efficiently produced by the method of producing apositive electrode active material described above.

The coefficient of variation in detection amounts of boron element on across-sectional surface of the positive electrode active materialparticles is preferably less than 0.18, more preferably 0.15 or less,still more preferably 0.14 or less, and particularly preferably 0.13 orless. The lower limit of the coefficient of variation is, for example,0.04 or more. The coefficient of variation in detection amounts of boronelement being the predetermined value or less is believed to indicatethat the boron compound is uniformly dispersed across the positiveelectrode active material particles.

An average of detection amounts of boron element to be used for thecalculation of the coefficient of variation is obtained by selecting anythree regions in any cross-sectional surface of the secondary particlescontained in the positive electrode active material, and calculating anarithmetic mean of the detection amounts in the selected regions. Astandard deviation of the detection amounts of boron element iscalculated from the average and the detection amounts in the regions.The detection amounts of boron element on a cross-sectional surface ofthe positive electrode active material particles may be measured byusing, for example, secondary ion mass spectrometer (SIMS).

The regions from which boron element is to be detected may be selectedfrom, for example, a region near the surface (surface section), a regionnear the center (center section), and a region between the surfacesection and the center section (middle section) in a cross-sectionalsurface of the positive electrode active material particles.Alternatively, multiple regions may be selected from each of the surfacesection, the middle section, and the center section, and an arithmeticmean of the detection amounts in the respective multiple regions may beused as the detection amount of each of the three regions.

The lithium transition metal composite oxide in the primary particleshas a composition ratio of the number of moles of nickel to the totalnumber of moles of metals other than lithium of from 0.7 to less than 1,preferably from 0.7 to 0.95, more preferably from 0.8 to 0.95, and stillmore preferably from 0.9 to 0.95. A positive electrode active materialcontaining a lithium transition metal composite oxide having a greaterratio of the number of moles of nickel can achieve good electricalcharge and discharge characteristics and cycle characteristics. Thecomposition of the lithium transition metal composite oxide may bedetermined using, for example, an inductively coupled plasma atomicemission spectrometer.

The lithium transition metal composite oxide may contain cobalt in itscomposition. When containing cobalt, the lithium transition metalcomposite oxide has a ratio of the number of moles of cobalt to thetotal number of moles of metals other than lithium of, for example, frommore than 0 to 0.3, and preferably from 0.02 to 0.2. The lithiumtransition metal composite oxide may contain manganese. When containingmanganese, the lithium transition metal composite oxide has a ratio ofthe number of moles of manganese to the total number of moles of metalsother than lithium of, for example, from more than 0 to 0.3, andpreferably from more than 0 to 0.15. The lithium transition metalcomposite oxide may contain aluminium. When containing aluminium, thelithium transition metal composite oxide has a ratio of the number ofmoles of aluminium to the total number of moles of metals other thanlithium of, for example, from more than 0 to 0.1, preferably from morethan 0 to 0.05, and more preferably from 0.01 to 0.04.

When the lithium transition metal composite oxide contains nickel,cobalt, and at least one of manganese and aluminium, the ratio ofnickel, cobalt, manganese and aluminium, Ni/Co/(Mn+Al), may be set to,for example, 8/1/1 or 8/1/(0.5+0.5) on a molar basis.

The lithium transition metal composite oxide may have a compositionrepresented by, for example, formula (1):Li_((1+p))Ni_(1−x−y−z−w)Co_(x)Mn_(y)Al_(z)M_(w)O₂  (1)

In the formula, −0.05≤p≤0.2, 0<x+y+z+w≤0.3, 0≤x≤0.3, 0≤y≤0.3, 0≤z≤0.1,and 0≤w≤0.03; and M is at least one selected from the group consistingof Zr, Ti, Mg, Ta, Nb, Mo, and W.

In the formula (1), in view of output characteristics, p is preferably−0.02 or more, or 0.02 or more, and p is also preferably 0.12 or less,or 0.06 or less. “x” is preferably from more than 0 to 0.3, and morepreferably from 0.02 to 0.2. “y” is preferably from more than 0 to 0.3,and more preferably from more than 0 to 0.15. “z” is preferably frommore than 0 to 0.1, more preferably from more than 0 to 0.05, and stillmore preferably from 0.01 to 0.04.

A compound containing boron is attached on at least a part of thesurfaces of the primary particles. An example of the boron-containingcompound is lithium metaborate (LiBO₂). The boron-containing compoundmay form a complex with the lithium transition metal composite oxide.The amount of the boron-containing compound in the positive electrodeactive material is, for example, from 0.1% by mole to 2% by mole, andpreferably from 0.1% by mole to 1.5% by mole in terms of a ratio of thenumber of moles of boron element to the total number of moles of metalsother than lithium in the lithium transition metal composite oxide. Theamount of boron in the positive electrode active material may bemeasured using, for example, an inductively coupled plasma atomicemission spectrometer.

A compound containing sodium is present in at least a part of theparticle boundaries of the secondary particles. An example of thesodium-containing compound includes sodium sulfate (Na₂SO₄). The amountof the sodium-containing compound in the positive electrode activematerial is, for example, in a range of from 100 ppm to 1400 ppm,preferably from 150 ppm to 1300 ppm, more preferably from 150 ppm to1200 ppm, still more preferably 200 ppm to 1000 ppm, and particularlypreferably from 300 ppm to 1000 ppm in terms of sodium element. Theamount of sodium element in the positive electrode active material maybe measured using, for example, an inductively coupled plasma atomicemission spectrometer.

The specific surface area of the positive electrode active material is,for example, in a range of from 0.2 m²/g to 3.0 m²/g, and preferablyfrom 0.3 m²/g to 2.0 m²/g. The specific surface area of the positiveelectrode active material may be measured by BET method.

The positive electrode active material when used in a positive electrodeof a non-aqueous electrolyte secondary battery, can contribute toachieve good cycle characteristics of the battery. The positiveelectrode active material is to be contained in a positive electrodeactive material layer arranged on the current collector of a positiveelectrode. In other words, the present disclosure encompasses anelectrode containing the positive electrode active material for anon-aqueous electrolyte secondary battery, and a non-aqueous electrolytesecondary battery including the electrode.

Electrode for Non-Aqueous Electrolyte Secondary Battery

The electrode for a non-aqueous electrolyte secondary battery mayinclude a current collector and a positive electrode active materiallayer arranged on the current collector, the positive electrode activematerial layer containing the positive electrode active materialdescribed above or the positive electrode active material produced bythe production method described above. A non-aqueous electrolytesecondary battery including the electrode may exhibit good cyclecharacteristics.

A density of the positive electrode active material layer may be, forexample, in a range of from 2.6 g/cm³ to 3.9 g/cm³, preferably from 2.8g/cm³ to 3.8 g/cm³, more preferably from 3.1 g/cm³ to 3.7 g/cm³, andstill preferably from 3.2 g/cm³ to 3.6 g/cm³. The density of thepositive electrode active material layer may be calculated by dividingthe mass of the active material layer by the volume of the activematerial layer. Here, the density of the positive electrode activematerial layer may be adjusted by applying pressure after applying anelectrode composition described later on the current collector.

Examples of the material for the current collector include aluminum,nickel, and stainless steel. The positive electrode active materiallayer may be formed by mixing the positive electrode active material, aconductive material, a binder, and so forth with a solvent to obtain anelectrode composition, applying the electrode composition on the currentcollector, and drying and pressurizing the composition. Examples of theconductive material include natural graphite, artificial graphite, andacetylene black. Examples of the binder include polyvinylidenedifluoride, polytetrafluoroethylene, and polyamide acrylic resin.Examples of the solvent include N-methyl-2-pyrrolidone (NMP).

Non-Aqueous Electrolyte Secondary Battery

The non-aqueous electrolyte secondary battery includes the electrode fora non-aqueous electrolyte secondary battery. The non-aqueous electrolytesecondary battery includes, in addition to the electrode for anon-aqueous electrolyte secondary battery, a negative electrode for anon-aqueous secondary battery, a non-aqueous electrolyte, and aseparator. For the negative electrode, the non-aqueous electrolyte, theseparator, and so forth to be included in the non-aqueous electrolytesecondary battery, those for a non-aqueous electrolyte secondary batterydescribed, for example, in Japanese Patent Application Publications No.2002-075367, No. 2011-146390, and No. 2006-12433, which are incorporatedin their entirety in the present specification, may be used asappropriate.

EXAMPLES

Examples of the present invention will now be described; however, thepresent invention is not limited to these Examples. A volume meandiameter is a value corresponding to 50% volume accumulation from thesmall particle diameter side in a volume distribution obtained using alaser scattering method. Specifically, a laser diffraction particle sizedispersion analyzer (Mastersizer 2000 by Malvern) was used to measurevolume mean particle diameters. Specific surface areas were measured bythe gaseous nitrogen adsorption method (one-point method) using a BETspecific surface area measuring device (Macsorb by Mountek). Alkalinecomponents were measured by adding the positive electrode activematerial to pure water, and titrating eluted lithium with sulfuric aciduntil a second point of neutralization is reached. The amount ofalkaline component neutralized with sulfuric acid was determined as theamount of lithium hydroxide (LiOH). The composition was measured usingan inductively coupled plasma atomic emission spectrometer (ICP-AES byPerkinElmer). The amount of sulfuric acid was measured using aninductively coupled plasma atomic emission spectroscopy (ICP-AES byHitachi). The amount of sodium (amount of Na) was measured using anatomic absorption spectrometer (AAS by Hitachi).

Example 1

Precursor Provision Step

Composite oxide particles having a composition represented by(Ni_(0.95)Co_(0.05))O₃ and a volume mean particle diameter of thesecondary particles of 20 μm was obtained by the coprecipitation method.

Synthesis Step

The resultant composite oxide particles, lithium hydroxide, and aluminumhydroxide were mixed in a molar ratio of Li:(Ni+Co):Al=1.10:0.97:0.03 toobtain a raw material mixture. The resultant raw material mixture washeat-treated in the air at a first temperature of 450° C. for 3 hours,and at a second temperature of 680° C. for 4 hours. Afterheat-treatment, the mixture underwent dispersion treatment to obtainfirst particles containing the lithium transition metal composite oxidewith a composition represented byLi_(1.03)Ni_(0.92)Co_(0.05)Al_(0.03)O₂.

Washing Step

The resultant first particles were added to an aqueous solution ofsodium sulfate with a sodium ion concentration of 0.469 mol/L to obtaina slurry with a solid concentration of 45% by mass. The solidconcentration was obtained by: the mass of the first particles/(the massof the first particles+the mass of the washing liquid). The slurry wasstirred for 30 min, dewatered through a funnel, and separated as a cake.The separated cake was dried at 150° C. for 10 hours to obtain secondparticles as washed particles.

Mixing Step

To the resultant second particles, orthoboric acid was added in anamount of 1% by mole in terms of boron element relative to the totalnumber of moles of metals other than lithium in the lithium transitionmetal composite oxide contained in the second particles, and mixed toobtain a mixture.

Heat Treatment Step

The resultant mixture was heat-treated at 250° C. for 10 hours in theair to obtain a positive electrode active material E1 containing atarget lithium transition metal composite oxide.

Example 2

A positive electrode active material E2 was obtained in the same or asimilar manner as in Example 1 except that the aqueous solution ofsodium sulfate in the washing step was replaced with an aqueous solutionof sodium sulfate with a sodium ion concentration of 0.156 mol/L.

Example 3

A positive electrode active material E3 was obtained in the same or asimilar manner as in Example 1 except that an aqueous solution of sodiumhydroxide was used in place of the aqueous solution of sodium sulfate inthe washing step.

Comparative Example 1

A positive electrode active material C1 was obtained in the same or asimilar manner as in Example 1 except that pure water was used in placeof the aqueous solution of sodium sulfate in the washing step, and thatthe solid concentration of the slurry was changed to 42% by mass.

Comparative Example 2

A positive electrode active material C2 was obtained in the same or asimilar manner as in Example 1 except that an aqueous solution oflithium sulfate with a lithium ion concentration of 0.469 mol/L was usedin place of the aqueous solution of sodium sulfate in the washing step,and that the solid concentration of the slurry was changed to 32% bymass.

Cycle Characteristics Evaluation 1

The positive electrode active materials obtained in Examples 1 to 3 andComparative Examples 1 and 2 were evaluated for cycle characteristics asdescribed below.

Provision of Positive Electrode

96.5 parts by mass of the positive electrode active material, 65 partsby mass of Super-C (by Timical), and 2 parts by mass of polyvinylidenedifluoride (PVDF) were dispersed and dissolved in N-methyl-2-pyrrolidone(NMP) to provide a positive electrode slurry. The positive electrodeslurry was applied on an aluminium foil current collector, and dried.After drying, a positive electrode was obtained by compression moldingwith a roll press machine so that the density of the positive electrodeactive material layer was 3.5 g/cm³, and cutting into a size of 15 cm².

Provision of Non-Aqueous Electrolyte

Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methylcarbonate (EMC) were mixed in a volume ratio of 3:4:3 to obtain a mixedsolvent. Into the mixed solvent, lithium hexafluorophosphate (LiPF₆) wasdissolved at a concentration of 1 mol/L to obtain a non-aqueouselectrolyte.

Fabrication of Battery for Evaluation

To the current collector of the positive electrode, a lead electrode wasconnected, and then vacuum-dried at 120° C. A separator formed fromporous polyethylene was arranged to cover the positive electrode in adry box, and packed in a laminated pouch pack. After packing, the pouchpack was vacuum-dried at 60° C. to remove moisture adsorbed on themembers. After vacuum drying, an Li foil adhered to a SUS plate in anargon box and the current collector of the positive electrode covered bythe separator were arranged in a manner to face each other, and placedinto the laminated pouch pack. The non-aqueous electrolyte describedabove was introduced into the laminated pouch pack, and the pouch packwas sealed to obtain a laminated non-aqueous electrolyte secondarybattery as a battery for evaluation.

Aging

The resultant battery for evaluation was electrically charged anddischarged once by constant-voltage/constant-current charging at acharge voltage of 4.25 V (counter electrode: Li) and a charge current of0.2 C (1 C≡a current at which discharge is completed in one hour), andconstant current discharging at a discharge voltage of 2.75 V (counterelectrode: Li) and a discharge current of 0.2 C.

Capacity Maintenance Measurement

After aging, discharge capacity after each cycle of n cycles wasmeasured at a constant temperature of 45° C. where one cycle consists ofconstant-voltage/constant-current charging at a charge voltage of 4.25 V(counter electrode: Li) and a charge current of 0.3 C, and constantcurrent discharging at a discharge voltage of 2.75 V (counter electrode:Li) and a discharge current of 0.3 C. The capacity maintenance rateafter n cycles Rs(n) expressed as a ratio≡Ed(n)/Ed(1) where Ed(n) is adischarge capacity after n cycles, and Ed(1) is a discharge capacityafter one cycle was calculated with the number of cycles n=30. Theevaluation results are shown in Table 1.

TABLE 1 Fabrication requirements Specific Washing liquid surface Na⁺Slurry area of Sulfuric Capacity Orthoboric concen- concen- second LiOHacid Na mainte- Composition acid tration tration particle contentcontent content nance Li Ni Co Al (mol %) Solute (mol/L) (mass %) (m²/g)(mass %) (mass %) (ppm) ratio (%) Example 1 1.03 0.92 0.05 0.03 1.0Sodium 0.469 45 1.31 0.52 0.20 710 93 sulfate Example 2 1.03 0.92 0.050.03 Sodium 0.156 45 1.51 0.57 0.11 230 91 sulfate Example 3 1.03 0.920.05 0.03 Sodium 0.469 45 1.34 0.47 0.06 980 97 hydroxide Comaparative1.03 0.92 0.05 0.03 — — 42 1.80 0.56 0.07 20 83 Example 1 Comaparative1.03 0.92 0.05 0.03 Lithium — 32 1.49 0.61 0.19 20 87 Example 2 sulfate

Table 1 shows the evaluation results of the batteries for evaluationcontaining the positive electrode active materials E1 to E3 of Examples1 to 3. These batteries were provided by the steps of washing with anaqueous solution of sodium salt, mixing a boron compound, andheat-treating the mixture. These batteries had evaluation results ofgood cycle characteristics. In contrast, the secondary particles of thepositive electrode active material C1 of Comparative Example 1 had alarge specific surface area. This is seemingly partly attributable towashing with pure water. The battery for evaluation containing thepositive electrode active material C1 showed a low capacity maintenancerate and poor battery performance. The secondary particles of thepositive electrode active material C2 of Comparative Example 2 had asmaller specific surface area than Comparative Example 1. This isseemingly partly attributable to washing with an aqueous solution oflithium sulfate. The battery for evaluation containing the positiveelectrode active material C2 showed a low capacity maintenance rate andpoor battery performance.

Example 4

Precursor Provision Step

Composite oxide particles having a volume mean particle diameter of 18μm, and a composition represented by (Ni_(0.85)Co_(0.15))O₃ was obtainedby the coprecipitation method.

Synthesis Step

First particles were obtained in the same or a similar manner as inExample 1 except that the mixing ratio in the synthesis step was changedto Li:(Ni+Co):Al=1.10:0.96:0.04 and that the second temperature waschanged to 745° C.

Washing Step

Second particles were obtained in the same or a similar manner as inExample 1 except that the aqueous solution of sodium sulfate in thewashing step was replaced with an aqueous solution of sodium sulfatewith a sodium ion concentration of 0.313 mol/L, and that the solidconcentration of the slurry was changed to 40% by mass.

Mixing Step

A positive electrode active material E4 was obtained in the same or asimilar manner as in Example 1 except that the amount of boron added inthe mixing step was changed to 0.3% by mole in terms of boron elementrelative to the total number of moles of metals other than lithium inthe lithium transition metal composite oxide contained in the secondparticles.

Comparative Example 3

The positive electrode active material C3 was obtained in the same or asimilar manner as in Example 4 except that pure water was used in placeof the aqueous solution of sodium sulfate used in the washing step, andthat the solid concentration of the slurry was changed to 45% by mass.

Comparative Example 4

The positive electrode active material C4 was obtained in the same or asimilar manner as in Example 4 except that an aqueous solution oflithium sulfate with a lithium ion concentration of 0.313 mol/L was usedin place of the aqueous solution of sodium sulfate used in the washingstep.

Each of the positive electrode active materials obtained in Example 4and Comparative Examples 3 and 4 were evaluated in the same or a similarmanner as in Cycle Characteristics Evaluation 1. The evaluation resultsare shown in Table 2.

TABLE 2 Fabrication requirements Specific Washing liquid surface Na⁺Slurry area of Sulfuric Capacity Orthoboric concen- concen- second LiOHacid Na mainte- Composition acid tration tration particle contentcontent content nance Li Ni Co Al (mol %) Solute (mol/L) (mass %) (m²/g)(mass %) (mass %) (ppm) ratio (%) Example 4 1.02 0.82 0.14 0.04 0.3Sodium 0.313 40 1.08 0.28 0.14 350 96 sulfate Comaparative 1.02 0.820.14 0.04 — — 45 1.51 0.27 0.07 30 90 Example 3 Comaparative 1.02 0.820.14 0.04 Lithium — 40 1.15 0.30 0.15 30 89 Example 4 sulfate

As shown in Table 2, the batteries for evaluation containing thepositive electrode active materials C3 and C4 of Comparative Examples 3and 4 showed a lower capacity maintenance rate and poorer batteryperformance than the battery for evaluation containing the positiveelectrode active material E4 of Example 4, which was washed with anaqueous solution of sodium salt.

Example 5

Precursor Provision Step

Composite oxide particles having a composition represented by(Ni_(0.88)Co_(0.09)Mn_(0.03))O₃ and a volume mean particle diameter of22 μm were obtained by the coprecipitation method.

Synthesis Step

First particles were obtained in the same or a similar manner as inExample 1 except that the mixing ratio in the synthesis step was changedto Li:(Ni+Co+Mn):Al=1.12:0.98:0.02, and that the second temperature waschanged to 730° C.

Washing Step

Second particles were obtained by washing the first particles with anaqueous solution of sodium sulfate with a sodium ion concentration of0.469 mol/L in the same or a similar manner as in Example 1.

Mixing Step

A mixture was obtained in the same or a similar manner as in Example 1except that the amount of boron added in the mixing step was changed to0.5% by mole in terms of boron element relative to the total number ofmoles of metals other than lithium of the lithium transition metalcomposite oxide contained in the second particles, and that tungstenoxide was added in an amount of 0.3% by mole relative to the totalnumber of moles of metals other than lithium in the lithium transitionmetal composite oxide contained in the second particles.

Heat Treatment Step

A positive electrode active material E5 was obtained by heat-treatingthe mixture in the same or a similar manner as in Example 1.

Comparative Example 5

A positive electrode active material C5 was obtained in the same or asimilar manner as in Example 5 except that pure water was used in placeof the aqueous solution of sodium sulfate in the washing step, and thatthe solid concentration of the slurry was changed to 37% by mass.

Comparative Example 6

A positive electrode active material C6 was obtained in the same or asimilar manner as in Example 5 except that an aqueous solution oflithium sulfate with a lithium ion concentration of 0.469 mol/L was usedin place of the aqueous solution of sodium sulfate used in the washingstep, and that the solid concentration of the slurry was changed to 28%by mass.

Cycle Characteristics Evaluation 2

The positive electrode active materials obtained in Example 5 andComparative Examples 5 and 6 were evaluated for cycle characteristics asdescribed below.

Provision of Positive Electrode Active Material

A positive electrode active material A containing a lithium transitionmetal composite oxide having a composition represented byLi_(1.03)Ni_(0.835)Co_(0.14)Al_(0.025)O₂ and a volume mean particlediameter of 4.5 μm, and obtained through the washing step with anaqueous solution of sodium sulfate was provided. The positive electrodeactive materials E5, C5, and C6 obtained in Example 5 and ComparativeExamples 5 and 6 were each mixed with the positive electrode activematerial A in a weight ratio of 7:3 to provide a mixed positiveelectrode active material for evaluation.

Fabrication of Positive Electrode

92 parts by mass of each of the mixed positive electrode activematerials obtained above description, 3 parts by mass of acetyleneblack, and 5 parts by mass of polyvinylidene difluoride (PVDF) weredispersed and dissolved in N-methyl-2-pyrrolidone (NMP) to provide apositive electrode slurry. The resultant positive electrode slurry wasapplied to an aluminium foil current collector and dried. After drying,a positive electrode was obtained by compression molding with a rollpress machine so that the density of the positive electrode activematerial layer was 3.5 g/cm³, and cutting into a size of 15 cm².

Fabrication of Negative Electrode

97.5 parts by mass of artificial graphite, 1.5 parts by mass ofcarboxymethyl cellulose (CMC), and 1.0 part by mass of styrene-butadienerubber (SBR) were dispersed in water to provide a negative electrodeslurry. The resultant negative electrode slurry was applied on a copperfoil, dried, and further compression-molded to obtain a negativeelectrode.

Provision of Non-Aqueous Electrolyte

Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in avolume ratio of 3:7 to obtain a mixed solvent. Into the mixed solvent,lithium hexafluorophosphate (LiPF₆) was dissolved at a concentration of1 mol/L to obtain a non-aqueous electrolyte.

Fabrication of Battery for Evaluation

Batteries for evaluation were fabricated in the same or a similar manneras in Cycle Characteristics Evaluation 1. Specifically, lead electrodeswere attached to the positive and negative electrode current collectors,respectively, and then vacuum drying was performed at 120° C.Subsequently, the separator was placed between the positive electrodeand the negative electrode, and they were stored into the laminatedpouch pack. After the storage, it was vacuum dried at 60° C. to removethe water adsorbed on each member. After vacuum drying, the non-aqueouselectrolyte was introduced into the laminated pouch pack, and the pouchpack was sealed to have a laminated non-aqueous electrolyte secondarybattery as a battery for evaluation.

Aging

The resultant batteries for evaluation were charged and discharged onceby constant-voltage/constant-current charging at a charge voltage of 4.2V (counter electrode C) and a charge current of 0.1 C, and constantcurrent discharging at a discharge voltage of, 2.75 V (counter electrodeC) and a discharge current of 0.2 C. Subsequently, the charge currentwas changed to 0.2 C, and charge and discharge was carried out twice toapply the non-aqueous electrolyte on the positive and negativeelectrodes well.

Discharge capacity maintenance rate was measured in the same or asimilar manner as in Cycle Characteristics Evaluation 1 except that thecharge voltage was changed to 4.2 V (counter electrode C), that thedischarge voltage was changed to 2.75 V (counter electrode C), and thatthe cycle number was changed to n=100. The evaluation results are shownin Table 3.

TABLE 3 Fabrication requirements Composition Tungsten Orthoboric Washingliquid Li Ni Co Mn Al oxide (mol %) acid (mol %) Solute Example 5 1.040.86 0.09 0.03 0.02 0.3 0.5 Sodium sulfate Comaparative 1.03 0.86 0.090.03 0.02 — Example 5 Comaparative 1.04 0.86 0.09 0.03 0.02 LithiumExample 6 sulfate Fabrication requirements Specific Washing liquidsurface Na⁺ Slurry area of Sulfuric Capacity concen- concen- second LiOHacid Na mainte- tration tration particle content content content nance(mol/L) (mass %) (m²/g) (mass %) (mass %) (ppm) ratio (%) Example 50.469 45 1.17 0.31 0.14 710 94 Comaparative — 37 1.47 0.34 0.04 20 90Example 5 Comaparative — 28 1.22 0.38 0.20 20 92 Example 6Evaluation of Boron Element Dispersion

The positive electrode active materials obtained in Example 5 andComparative Examples 5 and 6 were evaluated for boron element-dispersioninside the particles. Specifically, positive electrodes were eachfabricated in the same or a similar manner as described above, and thepositive electrodes were processed in a vacuum condition using an ionmilling device (IM400 Plus by Hitachi) to obtain a cross-sectionalsurface sample for each of some of the positive electrode activematerial particles. For the processing, Ar beam was used, and theprocessing time was 1 hour. The detection amount of each element in eachcross-sectional surface of the positive electrode active materialparticles of the cross-sectional surface samples was measured using adouble-focusing magnetic-sector mass analyzer (NanoSIMS 50L by Cameca).Using Cs⁺ primary ion species and a primary acceleration voltage of 8kV, signals of BO²⁻ which is a secondary ion (mass number 42.97) weremeasured by applying −8 kV onto the stage, and irradiating the sampleswith Cs⁺. Based on the intensity of the measured signals,cross-sectional surface images were produced. FIG. 1 is an example imageshowing BO²⁻ dispersion in a positive electrode active material particleof Example 5. FIG. 2 is an example image showing BO²⁻ dispersion of apositive electrode active material particle of Comparative Example 5.FIG. 3 is an example image showing BO²⁻ dispersion of a positiveelectrode active material particle of Comparative Example 6.

From the image data, the amounts of BO²⁻ detected in the area near thesurface of the particles (surface section), the area in the middle partof the particles (middle section), and the area around the centersection of the particles (center section) were analytically calculatedusing an image analysis software (OpenMIMS ImageJ Plugin). The analysiswas conducted by choosing two regions of the surface section, tworegions of the middle section, and two regions of the center section forthree particles from each of the positive electrode active materialparticles of Examples and Comparative Examples. Each of the regions wasselected in a manner approximately symmetrical to one another on a linecrossing the cross-sectional surface with respect to the center of aparticle, each region having an area of about 1.5×10⁻¹¹ m². The tworegions in the center section were selected as one continuous region.For the detection amounts of BO²⁻ on the surface section and the middlesection, an arithmetic mean of the respective two regions were obtained.An arithmetic mean value t1, a standard deviation σ1, and a coefficientof variation (CV: σ1/t1) were calculated from the resultant detectionamounts for the detection amounts of BO²⁻ in the three regions, or thesurface section, the middle section and the center section, inside theparticles of the positive electrode active materials. The standarddeviation σ1 was calculated using STDEV.P function of EXCEL. For eachcoefficient of variation CV, an arithmetic mean of the three particlesof each of Examples and Comparative Examples were obtained, and theresults are shown in Table 4. These results were deemed as boronelement-dispersion, and used for evaluation.

TABLE 4 σ1/t1 Example 5 0.12 Comparative 0.18 Example 5 Comparative 0.21Example 6

As shown in Tables 3 and 4, the positive electrode active materials ofComparative Examples 5 and 6 had a lower capacity maintenance rate andpoorer battery performance than the positive electrode active materialof Example 5, which was washed with an aqueous solution of sodium salt.Also, the compound containing boron was less uniformly dispersed insidethe particles of the positive electrode active materials of ComparativeExamples 5 and 6 than the positive electrode active material of Example5. This is seemingly partly attributable to the absence of sodium in theparticle boundaries of the positive electrode active material particlesin the comparative examples.

Example 6

Precursor Provision Step

Composite oxide particles represented by (Ni_(0.885) Co_(0.115))O₃ andhaving a volume mean particle diameter of 4 μm were obtained by thecoprecipitation method.

Synthesis Step

Primary particles were obtained in the same or a similar manner as inExample 1 except that the mixing ratio in the synthesis step was changedto Li:(Ni+Co):Al=1.10:0.97:0.03, and that the second temperature waschanged to 690° C.

Washing Step

Second particles were obtained in the same or a similar manner as inExample 1 except that the aqueous solution of sodium sulfate in thewashing step was replaced with an aqueous solution of sodium sulfatewith a sodium ion concentration of 0.156 mol/L.

Mixing Step

A positive electrode active material E6 was obtained in the same or asimilar manner as in Example 1 except that the amount of boron added inthe mixing step was changed to 0.1% by mole in terms of boron elementrelative to the total number of moles of metals other than lithium inthe lithium transition metal composite oxide contained in the secondparticles.

Comparative Example 7

A positive electrode active material C7 was obtained in the same or asimilar manner as in Example 6 except that an aqueous solution oflithium sulfate with a lithium ion concentration of 0.156 mol/L was usedin place of the aqueous solution of sodium sulfate used in the washingstep, and that the solid concentration of the slurry was changed to 30%by mass.

The positive electrode active materials obtained in Example 6 andComparative Example 7 were evaluated for cycle characteristics asdescribed below.

A positive electrode active material B was provided which contained alithium transition metal composite oxide having a compositionrepresented by Li_(1.03)Ni_(0.86)Co_(0.09)Mn_(0.03)Al_(0.02)O₂ and avolume mean particle diameter of 22 μm, and was obtained through washingstep with an aqueous solution of sodium sulfate was provided.

The positive electrode active materials E6 and C7 obtained in Example 6and Comparative Example 7 were each mixed with the positive electrodeactive material B in a weight ratio of 3:7 to provide mixed positiveelectrode active materials for evaluation, and the mixed positiveelectrode active materials were evaluated as in Cycle CharacteristicsEvaluation 2. The evaluation results are shown in Table 5.

TABLE 5 Fabrication requirements Specific Washing liquid surface Na⁺Slurry area of Sulfuric Capacity Orthoboric concen- concen- second LiOHacid Na mainte- Composition acid tration tration particle contentcontent content nance Li Ni Co Al (mol %) Solute (mol/L) (mass %) (m²/g)(mass %) (mass %) (ppm) ratio (%) Example 6 1.03 0.86 0.11 0.03 0.1Sodium 0.156 45 2.15 0.36 0.15 410 96 sulfate Comaparative 1.03 0.860.11 0.03 Lithium — 30 2.16 0.37 0.28 40 95 Example 7 sulfate-

As shown in Table 5, the positive electrode active material ofComparative Example 7 showed a lower capacity maintenance rate andpoorer battery performance than the positive electrode active materialof Example 6.

Evaluation of Resistance Increase Rate

With respect to the positive electrode containing the positive electrodeactive material obtained by the same or a similar manufacturing methodas that obtained in Example 5 and Comparative Examples 5 and 6, theresistance increase rate was evaluated as follows.

Example 7

Provision of Positive Electrode Active Material

A positive electrode active material for evaluation was provided by thesame or a similar manufacturing method as in Example 5.

Fabrication of Positive Electrode

92 parts by mass of each of the mixed positive electrode activematerials obtained above, 3 parts by mass of acetylene black, and 5parts by mass of polyvinylidene difluoride (PVDF) were dispersed anddissolved in N-methyl-2-pyrrolidone (NMP) to provide a positiveelectrode slurry. The resultant positive electrode slurry was applied toan aluminium foil current collector and dried. After drying, a positiveelectrode of Example 7 was obtained by compression molding with a rollpress machine so that the density of the positive electrode activematerial layer was 2.8 g/cm³, and cutting into a size of 15 cm². Thedensity of the positive electrode active material layer was calculatedby dividing the mass of the positive electrode active material layer bythe volume of the positive electrode active material layer calculated bymeasuring the thickness of the positive electrode active material layerwith a micrometer.

Fabrication of Battery for Evaluation

Battery for evaluation was fabricated in the same or a similar manner asin Cycle Characteristics Evaluation 2 except for using the positiveelectrode obtained above.

Example 8

A positive electrode of Example 8 was obtained in the same or a similarmanner as in Example 7 except that compression molding was performed sothat the density of the positive electrode active material layer was 3.3g/cm³. Then, a battery for evaluation was obtained in the same or asimilar manner as in Example 7 except that this was used.

Example 9

A positive electrode of Example 9 was obtained in the same or a similarmanner as in Example 7 except that compression molding was performed sothat the density of the positive electrode active material layer was 3.5g/cm³. Then, a battery for evaluation was obtained in the same or asimilar manner as in Example 7 except that this was used.

Example 10

A positive electrode of Example 10 was obtained in the same or a similarmanner as in Example 7 except that compression molding was performed sothat the density of the positive electrode active material layer was 3.7g/cm³. Then, a battery for evaluation was obtained in the same or asimilar manner as in Example 7 except that this was used.

Comparative Example 8

A positive electrode of Comparative Example 8 was obtained in the sameor a similar manner as in Example 7 except that the positive electrodeactive material obtained by the same or a similar manufacturing methodas in Comparative Example 5 was used. Then, an evaluation battery wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 9

A positive electrode of Comparative Example 9 was obtained in the sameor a similar manner as in Comparative Example 8 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.3 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 10

A positive electrode of Comparative Example 10 was obtained in the sameor a similar manner as in Comparative Example 8 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.5 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 11

A positive electrode of Comparative Example 11 was obtained in the sameor a similar manner as in Comparative Example 8 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.7 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 12

A positive electrode of Comparative Example 12 was obtained in the sameor a similar manner as in Example 7 except that the positive electrodeactive material obtained by the same or a similar manufacturing methodas in Comparative Example 6 was used. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 13

A positive electrode of Comparative Example 13 was obtained in the sameor a similar manner as in Comparative Example 12 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.3 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 14

A positive electrode of Comparative Example 14 was obtained in the sameor a similar manner as in Comparative Example 12 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.5 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Comparative Example 15

A positive electrode of Comparative Example 15 was obtained in the sameor a similar manner as in Comparative Example 12 except that compressionmolding was performed so that the density of the positive electrodeactive material layer was 3.7 g/cm³. Then, a battery for evaluation wasobtained in the same or a similar manner as in Example 7 except thatthis was used.

Aging

The resultant batteries for evaluation were electrically charged anddischarged once by constant-voltage/constant-current charging at acharge voltage of 4.2 V (counter electrode C) and a charge current of0.1 C, and constant current discharging at a discharge voltage of, 2.75V (counter electrode C) and a discharge current of 0.2 C. Subsequently,the charge current was changed to 0.2 C, and charge and discharge wascarried out twice to apply the non-aqueous electrolyte on the positiveand negative electrodes well.

Measurement of AC Impedance

After aging, the batteries for evaluation were charged to a state ofcharge (SOC) of 100% by constant-voltage/constant-current charging at acharging voltage of 4.2V and a charging current of 0.2 C. Using animpedance measuring device (1470E and 1455A, both manufactured bySOLARTRON Co., Ltd.), resistance measurements were performed in therange of 1 MHz to 0.1 Hz by the AC impedance method to obtain a Nyquistplot. After the above resistance measurement, constant current dischargewas performed at a discharge voltage of 2.75 V and a discharge currentof 0.2 C. Then, 1 cycle including constant-voltage/constant-currentcharging at a charging voltage of 4.2V and charging current of 1 C, andconstant current discharging at a discharging voltage of 2.75V anddischarge current of 1 C at a constant temperature of 45° C., and 200cycles of charging/discharging were performed. After 200 cycles ofcharging/discharging, the batteries for evaluation were charged to SOC100% by constant-voltage/constant-current charging at a charging voltageof 4.2V and a charging current of 0.2 C, and the resistance measurementswere performed in the same or a similar manner using the above impedancemeasuring device to obtain a Nyquist plot.

Calculation of Resistance Increase Rate

Based on the obtained Nyquist plot, the equivalent circuit model of FIG.4 was assumed and the fitting calculation was performed. The higher peakfrequency of the arc component of the impedance obtained by themeasurement was defined as the resistance derived from the negativeelectrode, and the lower peak frequency was defined as the resistance Rderived from the positive electrode. The resistance value derived fromthe positive electrode before the cycle was calculated as R(p), theresistance value derived from the positive electrode after the cycle wasdefined as R(a), and R(a)/R(p)×100(%) was calculated as the resistanceincrease rate. Then, as in Comparative Example 8 with respect to Example7, the resistance increase rate in the Example or Comparative Example inwhich the positive electrode active material was washed with thecleaning liquid was divided by the resistance increase rate in theComparative Example in which the positive electrode active material waswashed with pure water and the positive electrode active material layerwas provided with the same or a similar density to calculate a relativeresistance increase ratio. Table 6 shows the evaluation results forExamples 7 to 10 washed with an aqueous sodium sulfate solution andComparative Examples 12 to 15 washed with an aqueous lithium sulfatesolution.

TABLE 6 Relative Solute of resistance washing Density increase luquid(g/cm³) ratio Example 7 Sodium 2.8 0.552 sulfate Comparative Lithium 2.80.708 Example 12 sulfate Example 8 Sodium 3.3 0.455 sulfate ComparativeLithium 3.3 0.723 Example 13 sulfate Example 9 Sodium 3.5 0.471 sulfateComparative Lithium 3.5 0.725 Example 14 sulfate Example 10 Sodium 3.70.641 sulfate Comparative Lithium 3.7 0.808 Example 15 sulfate

As shown in Table 6, at any density of positive electrode activematerial layer, the relative resistance increase ratio in the positiveelectrode of the Example containing the positive electrode activematerial obtained through the washing step using the salt aqueoussolution containing sodium ions was low and the deterioration of theoutput after the cycle was suppressed. From these results, it wasconfirmed that, not only in the capacity maintenance ratio, but also inresistance, the characteristics of the non-aqueous electrolyte secondarybattery containing the positive electrode active material obtainedthrough the washing step by using the salt aqueous solution containingsodium ions are improved.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

Although the present disclosure has been described with reference toseveral exemplary embodiments, it is to be understood that the wordsthat have been used are words of description and illustration, ratherthan words of limitation. Changes can be made within the purview of theappended claims, as presently stated and as amended, without departingfrom the scope and spirit of the disclosure in its aspects. Although thedisclosure has been described with reference to particular examples,means, and embodiments, the disclosure may be not intended to be limitedto the particulars disclosed; rather the disclosure extends to allfunctionally equivalent structures, methods, and uses such as are withinthe scope of the appended claims.

One or more examples or embodiments of the disclosure may be referred toherein, individually and/or collectively, by the term “disclosure”merely for convenience and without intending to voluntarily limit thescope of this application to any particular disclosure or inventiveconcept. Moreover, although specific examples and embodiments have beenillustrated and described herein, it should be appreciated that anysubsequent arrangement designed to achieve the same or a similar purposemay be substituted for the specific examples or embodiments shown. Thisdisclosure may be intended to cover any and all subsequent adaptationsor variations of various examples and embodiments. Combinations of theabove examples and embodiments, and other examples and embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the description.

In addition, in the foregoing Detailed Description, various features maybe grouped together or described in a single embodiment for the purposeof streamlining the disclosure. This disclosure may be not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter may bedirected to less than all of the features of any of the disclosedembodiments. Thus, the following claims are incorporated into theDetailed Description, with each claim standing on its own as definingseparately claimed subject matter.

The above disclosed subject matter shall be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure may bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

What is claimed is:
 1. A method of producing a positive electrode activematerial, the method comprising: contacting first particles that containa lithium transition metal composite oxide with a solution containingsodium ions to obtain second particles containing the lithium transitionmetal composite oxide and sodium element, wherein the lithium transitionmetal composite oxide has a layered structure and a composition ratio ofa number of moles of nickel to a total number of moles of metals otherthan lithium in a range of from 0.7 to less than 1; mixing the secondparticles and a boron compound to obtain a mixture; and heat-treatingthe mixture at a temperature in a range of from 100° C. to 450° C. 2.The method according to claim 1, wherein the second particles have asodium element content in a range of from 100 ppm to 1400 ppm.
 3. Themethod according to claim 2, wherein the mixture contains the boroncompound in an amount of from 0.1% by mole to 2% by mole in terms of aratio of a number of moles of boron element relative to the total numberof moles of metals other than lithium in the lithium transition metalcomposite oxide.
 4. The method according to claim 3, wherein the lithiumtransition metal composite oxide contains cobalt and has a ratio of anumber of moles of cobalt to the total number of moles of metals otherthan lithium in the lithium transition metal composite oxide of 0.3 orless.
 5. The method according to claim 4, wherein the lithium transitionmetal composite oxide contains at least one of manganese and aluminiumand has a ratio of a number of moles of manganese and aluminium to thetotal number of moles of metals other than lithium in the lithiumtransition metal composite oxide of 0.3 or less.
 6. The method accordingto claim 1, wherein the mixture contains the boron compound in an amountof from 0.1% by mole to 2% by mole in terms of a ratio of a number ofmoles of boron element relative to the total number of moles of metalsother than lithium in the lithium transition metal composite oxide. 7.The method according to claim 1, wherein the lithium transition metalcomposite oxide contains cobalt and has a ratio of a number of moles ofcobalt to the total number of moles of metals other than lithium in thelithium transition metal composite oxide of 0.3 or less.
 8. The methodaccording to claim 1, wherein the lithium transition metal compositeoxide contains at least one of manganese and aluminium and has a ratioof a number of moles of manganese and aluminium to the total number ofmoles of metals other than lithium in the lithium transition metalcomposite oxide of 0.3 or less.
 9. The method according to claim 1,wherein the lithium transition metal composite oxide has a compositionrepresented by formula (1):Li_((1+p))Ni_((1−x−y−z−w))Co_(x)Mn_(y)Al_(z)M_(w)O₂  (1) wherein−0.05≤p≤0.2, 0<x+y+z+w≤0.3, 0≤x≤0.3, 0≤y≤0.3, 0≤z≤0.1, and 0≤w≤0.03; andM is at least one selected from the group consisting of Zr, Ti, Mg, Ta,Nb, Mo, and W.